The field of the disclosure relates generally to high voltage direct current (HVDC) transmission systems and, more particularly, to HVDC converter systems and a method of operation thereof.
At least some of known electric power generation facilities are physically positioned in a remote geographical region or in an area where physical access is difficult. One example includes power generation facilities geographically located in rugged and/or remote terrain, for example, mountainous hillsides, extended distances from the customers, and off-shore, e.g., off-shore wind turbine installations. More specifically, these wind turbines may be physically nested together in a common geographical region to form a wind turbine farm and are electrically coupled to a common alternating current (AC) collector system. Many of these known wind turbine farms include a separated power conversion assembly, or system, electrically coupled to the AC collector system. Such known separated power conversion assemblies include a rectifier portion that converts the AC generated by the power generation facilities to direct current (DC) and an inverter that converts the DC to AC of a predetermined frequency and voltage amplitude. The rectifier portion of the separated power conversion assembly is positioned in close vicinity of the associated power generation facilities and the inverter portion of the separated full power conversion assembly is positioned in a remote facility, such as a land-based facility. Such rectifier and inverter portions are typically electrically connected via submerged high voltage direct current (HVDC) electric power cables that at least partially define an HVDC transmission system.
Many known power converter systems include rectifiers that include line commutated converters (LCCs). LCC-based rectifiers typically use thyristors for commutation to “chop” three-phase AC voltage through firing angle control to generate a variable DC output voltage. Commutation of the thyristors requires a stiff, i.e., substantially nonvarying, grid voltage. Therefore, for those regions without a stiff AC grid, converters with such rectifiers cannot be used. Also, a “black start” using such a HVDC transmission system is not possible. Further, such known thyristor-based rectifiers require significant reactive power transmission from the AC grid to the thyristors, with some reactive power requirements representing approximately 50% to 60% of the rated power of the rectifier. Moreover, thyristor-based rectifiers facilitate significant transmission of harmonic currents from the AC grid, e.g., the 11th and 13th harmonics, such harmonic currents typically approximately 10% of the present current loading for each of the 11th and 13th harmonics. Therefore, to compensate for the harmonic currents and reactive power, large AC filters are installed in the associated AC switchyard. In some known switchyards, the size of the AC filter portion is at least 3 times greater than the size of the associated thyristor-based rectifier portion. Such AC filter portion of the switchyard is capital-intensive due to the land required and the amount of large equipment installed. In addition, a significant investment in replacement parts and preventative and corrective maintenance activities increases operational costs.
In addition, many known thyristors in the rectifiers switch only once per line cycle. Therefore, such thyristor-based rectifiers exhibit operational dynamic features that are less than optimal for generating smoothed waveforms. Also, typically, known thyristor-based LCCs are coupled to a transformer and such transformer is constructed with heightened ratings to accommodate the reactive power and harmonic current transmission through the associated LCC. Moreover, for those conditions that include a transient, or fault, on either of the AC side and the DC side of the thyristor-based rectifier, interruption of proper commutation may result.